CRC LEME OPEN FILE REPORT 164

A L Herczeg

October 2004

SUMMARY

Groundwaters of the Lower Balonne region of south-western Queensland are of marginal to brackish quality with a dominantly seawater-like composition. Twenty four groundwater samples from the two main Cainozoic alluvial aquifers, upper alluvial (UA) and lower alluvial (LA), the middle confining bed (MCB) and the Cretaceous Griman Creek meta-sedimentary formation (GCF) were collected in August-September 2003 and analysed for major and minor ion chemistry as well as a suite of environmental isotopes.

Sodium and chloride ions make up >85% of the salt load in the groundwaters with SO 4 2- , Ca 2+ and Mg 2+ making up most of the remaining. Cl - /Br - mass ratios of the groundwaters are the same as seawater (270-290) indicating a marine origin for dissolved salts. The ratios are much less than that which would be derived from dissolution of halite or rock-forming minerals, which have Cl - /Br - >5,000; therefore marine aerosols deposition via rainfall is the dominant source of salt in all the main aquifers. The linear relationship between the dominant ions Na + and Cl - , with a slope of 0.56 identical to seawater, indicates evapo-transpiration is responsible for concentration of the ions prior to recharge. Isotope ratios of 34 S/ 32 S from dissolved sulphate are slightly less then seawater indicating marine sulphate and organic sulphur are the sources of dissolved sulphate in these groundwaters, rather than from dissolution of reduced sulphur minerals such as pyrite.

Recharge rates based on chloride mass balance calculations and median groundwater chloride concentrations indicate median recharge rates of 0.1 mm/yr for the Griman Creek formation, 0.25 mm/yr for the UA and about 0.5 mm/yr for the MCB and LA. However, individual recharge estimates range from 0.05 mm/yr for the highly saline UA and GCF waters to 5 mm/yr for the fresh groundwaters in the UA and MCB.

Stable isotope compositions of the groundwaters ( d 18 O and d 2 H) indicate a meteoric composition and form a trend to the right of the global meteoric water line on a d 18 O versus d 2 H plot with a slope of 5.8. This suggests that recharge is dominated by rainfall and evaporation has altered the isotopic composition prior to recharge.

Radiocarbon ages for the lower alluvial aquifer tend to be much older (>5,000 - >25,000 years) than the upper alluvial aquifer (<2,500 year) suggesting that vertical leakage occurs at a very slow rate. There is no apparent trend in ages of the lower alluvial aquifer from the NE to SW part of the study area. Fresh, relatively young (<50 years) groundwater may be restricted to near the rivers suggesting flood recharge is limited in lateral extent via surface tributaries, but further work would be required to confirm this. There is evidence for vertical exchange of the groundwaters amongst the various aquifer systems (indicated by chemistry and stable isotope data) but this occurs on time scale of 10 3 to 10 4 years as indicated by 'old' 14 C groundwater ages at depths >20 m below the water table.

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